Moving coil loudspeaker with eddy current suppression

ABSTRACT

An improved structure for a dynamic vibration generator, such as a loudspeaker or a microphone, which includes stacked sheets of high permeability material in the area of the pole gaps to suppress eddy currents.

United States Patent Inventor Appl. No. Filed Patented Priority PaulMauz Badstrasse 1, 724 Horb, Germany Mar. 24, 1970 Jan. 4, 1972 Feb. 17,1965 Germany Continuation of application Ser. No. 500,630, Oct.22, 1965,now abandoned. This application Mar. 24, 1970, Ser. No. 36,761

MOVING COIL LOUDSPEAKER WITH EDDY CURRENT SUPPRESSION 3 Claims, 1Drawing Fig.

US. Cl 179/119 R, 179/115.5 R, 335/231 Int. Cl -H04r 9/02 [50] FieldoISeareh 179/1 15.5

[56] References Cited UNITED STATES PATENTS 1,976,874 10/1934 Brzeski179/1 l5.5 OTHER REFERENCES Brailsford, Magnetic Materials, 1960,Methuen & Co., Chaps. lV&V

Bardell, Magnetic Materials in the Electrical Industry, 1960, MacDonald& Co., Chap. 4

Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas L. KundertAtmrney--Shlesinger, Shlesinger & Arkwright ABSTRACT: An improvedstructure for a dynamic vibration generator, such as a loudspeaker or amicrophone, which includes stacked sheets of high permeability materialin the area of the pole gaps to suppress eddy currents.

PATENTEU JAN 4 I972 ATTORNEY MOVING COIL LOUDSPEAKER WITH EDDY CURRENTSUPPRESSION This application is a continuation of application Ser. No.500,630, filed Oct. 22, 1965, and now abandoned.

This invention relates to dynamic vibration generators, and moreparticularly to dynamic loudspeakers having a permanent ring-shapedmagnet and having a vibratory coil connected with a diaphragm centeredin its cylindrical annular gap.

The application of the invention extends furthermore to dynamicmicrophones or dynamic sound systems of any kind. Such sound systemsusually consist of a ringlike magnet of any construction where acylindrical annular gap is provided between the central core and theannular plate surrounding the core. A vibratory coil with a conventionaltensioned diaphragm is centered in the annular gap.

The flux of the permanent magnet acts crosswise to the longitudinal axisof the vibratory coil in the annular gap of the magnet. The vibratorycoil connected for example to an output amplifier is fed withaudiofrequency currents, which create in the coil a magnet field whichextends perpendicularly to the field of the permanent magnet. Themagnetic reproduction field, continuously varying as a function of theaudiofrequency currents, pushes the coil alternately inward and outwardin relation to the ring magnet. By the connection of the vibratory coilwith a diaphragm, the audiofrequency electric vibrations are transformedinto mechanical sound vibrations by such an arrangement.

The known dynamic vibration systems, as used at present for loudspeakersand dynamic microphones, cannot satisfy higher requirements. It is knownthat by amplification of the magnetic flux in the annular gap thequality of reproduction. of loudspeakers can be improved. This isattributable to the fact that with a stronger magnetic field thevibratory coil is deflected with greater force or the deflectingmovement is delayed less. It has been observed, for example, that thereproduction volume increases proportionally with the increase inmagnetic field strength. The known measures for accommodating as greatas possible a magnetic field strength between the pole surfacesdetermined by the size of a magnet are limited by the saturation valuesof the iron.

For the improvement of the reproduction quality or frequency fidelity indynamic sound systems the present invention proceeds from a hithertoneglected, but influential process. The magnetic field of thecurrent-carrying vibratory coil, which moves in the field of thepermanent magnet, leads to reactions in the massive iron of thepermanent magnet. That is, the moving magnetic field of the vibratorycoil produces eddy currents in the poles in the zone of the gap. Thelosses due to the eddy currents weaken the magnetic field of thevibratory coil, so that therefore also the movement or the deflection ofthe vibratory coil from the field of the permanent magnet is impaired ordelayed, owing to which in turn the audiofrequency vibrations to betransformed are varied harmfully for the quality of reproduction.

It is known that electric vibrations of higher frequencies or themagnetic fields of the vibratory coil thus varying more rapidly producegreater eddy currents than the lower. This results in a vibrationmixture in which the audio vibrations of different pitch are delayed ordamped in different degree, so that a distortion of the acoustic patternoccurs.

The invention thus has the task of providing means whereby the eddycurrents in the area of the magnet poles which are attributable to thecurrent-carrying vibratory coil are eliminated or sufficiently reduced.The solution of this problem resides in that the poles on both sides ofthe annular gap consist of annular, low remanence dynamo sheets whichare stacked on one another in the longitudinal direction of the gap. Thesheets thus extend crosswise to the direction of movement of the coiland their outer or inner end faces are directed toward the coil. Thewidth of the sheets is evident from the particular operating data orfrom the strength of the audiofrequency currents and from the frequencyrange to be transmitted. As the lower vibrations in the vibrator coilbrin about a more widely expandrng magnetic field t an the hrg ervibrations, care must be taken-that also these low vibrations or themagnetic field produced by them remains unweakened in the poles of themagnet and that no eddy currents are produced.

Instead of dynamo sheets, the poles on both sides of the gap may consistof high-frequency iron, housed in corresponding annular cavities on bothsides of the air gap. The following may be considered high-frequencyiron: pulverized iron, iron chips, sintered iron, separately, pure ormixed, iron dust embedded in plastic, iron oxide compounds with othermetals which combine good permeability with poor conductivity.

According to another proposal of the invention, the central core of thering magnet contiguous to the pole region is recessed or reduced in itsdiameter. Thereby eddy current losses contiguous to the pole region areavoided, and at the same time the magnetic field is concentrated.

The figure illustrates the invention with respect to a few ex amples ofembodiment The FIGURE shows diagrammatically a loudspeaker according tothe invention.

According to the diagrammatic representation of a dynamic loudspeaker asset forth in the FIGURE, the magnetically energized circuit (cup-shapedmagnet) consists of a circular iron disc 1, a ring 2 formed for exampleof sintered magnet iron, and an iron pole ring 3 fitted thereupon whichsurrounds a core portion 4 which is secured by threads or by adhesivemeans to the circular disc 1. The structural parts I, 2 and 3 can alsobe secured to one another by adhesive means. The air gap of the magnet,in which the oscillation coil 5 of the loudspeaker is centered, isforrned of transformer sheet metal rings punched out in the form of twoconcentric rings 6 and 7, whereby the outer ring 6 is fastened in acorresponding recess in pole ring 3 whereas the inner ring 7 is fastenedin a corresponding recess in core portion 4. The oscillation coil 5 isconnected with the aid of conventional fastening means to the diaphragm8 of the loudspeaker, As a further embodiment, the iron core may berecessed, that is, its diameter may be recessed in relation to the partlying above or below.

The radial thickness or depth of the aforesaid means for suppressingeddy currents is smaller in the case of loudspeakers of smaller outputthan in the case of larger loudspeakers. As an average, the axial depthis about 5 millimeters.

I claim:

1. A dynamic vibration generator comprising:

a. a permanent magnet having a cylindrical annular air gap,

b. a vibratory coil in said annular gap,

c. a diaphragm connected to said vibratory coil,

d. poles on each side of said air gap having recesses therein,

e. means for suppressing eddy currents also positioned on each side ofsaid air gap adjacent said coil in said recesses,

f. said suppressing means being in the form of a stacked plurality offlat rings of dynamo sheet material having axes parallel to the path ofsaid vibratory coil and being positioned transversely to said vibratorycoil path.

2. A dynamic vibration generator as set forth in claim I wherein saidrecesses are formed by shoulders on the inner and outer circumferencesof respective poles for centering and supporting said dynamo sheets.

3. A dynamic vibration generator as defined in claim I wherein saidsuppressing means has a radial depth of approximately 5 millimeters.

1. A dynamic vibration generator comprising: a. a permanent magnethaving a cylindrical annular air gap, b. a vibratory coil in saidannular gap, c. a diaphragm connected to said vibratory coil, d. poleson each side of said air gap having recesses therein, e. means forsuppressing eddy currents also positioned on each side of said air gapadjacent said coil in said recesses, f. said suppressing means being inthe form of a staCked plurality of flat rings of dynamo sheet materialhaving axes parallel to the path of said vibratory coil and beingpositioned transversely to said vibratory coil path.
 2. A dynamicvibration generator as set forth in claim 1 wherein said recesses areformed by shoulders on the inner and outer circumferences of respectivepoles for centering and supporting said dynamo sheets.
 3. A dynamicvibration generator as defined in claim 1 wherein said suppressing meanshas a radial depth of approximately 5 millimeters.